A little theory first… The most common camera system in the world is based on 35mm film (the “type 135”), which is exactly the Kodak you shot with when you were film cameras. That is why this standard has become a benchmark for photo producers in the consumer sector, and it is about it that we will talk in the article below, “forgetting” that there is also a medium and large format.
Today’s lenses, one way or another, are tied to the 35-mm format, despite the fact that cameras today are farther from it than ever – less than 1% of all cameras that are guided by this standard are produced for a similar frame size (24 × 36 mm). . Moreover, the vast majority of manufactured cameras (more than 90%) are compact, in which the size of the photosensitive matrix is 4-6 times smaller than the area of the same film frame. And yet, in order to follow at least some standard, 35 mm film has become a benchmark for everyone.
If you look at the lens of a compact device, you can often see two scales on it, for example 8-24 mm / f2.8-5.0 (38-114 mm), where the designation in brackets corresponds to the focal length (note that this is not the size of the lens), recalculated in 35 mm equivalent. It is on this basis that lenses differ (all other parameters are not as important as they are said to be).
To understand this, let’s imagine two cameras: an old film “soap box” and a modern digital compact with a resolution of 10 megapixels. We shoot the same frame on both at a focal length of 38 mm from the same position and print it on a 10×15 format. Looking at the frame and not paying attention to the difference in quality, we understand that there are practically no differences in the space covered – and therefore, is there any difference for us that the matrix is 4 times smaller than the film frame, but in fact the focal length is only 8 mm ? That is why the focal length (FR) is simply multiplied by the appropriate coefficient (“crop factor”), which can be obtained by dividing the size of the film frame diagonally by the corresponding size of the matrix – this is the effective focal length.
However, for DSLRs the situation is reversed: sincerely believing that their owners are extremely experienced people who understand the essence of the issue, none of the lens manufacturers for DSLRs bother to calculate the EGF for them. Meanwhile, there are not so many full-frame devices – mostly professional models, of which no more than a dozen have been released today by all manufacturers – and the vast majority of DSLRs existing today have a matrix 1.5 times smaller, for which a lot of lenses have already been released that do not support full frame coverage . Nevertheless, the indication of focal lengths even on them remained standard for “reflex” lenses – the user simply needs to multiply the focal length by the corresponding crop factor (×1.5) in each case. By the way, if you crop a picture in the editor, you also change the EGF (if it is, of course, interesting) – after all, you print the picture on paper of the same size or watch it on the network at the same monitor resolution … For example, if you shot a 50 mm frame, multiply by 1.5 (crop factor), crop, for example, by 30% and, accordingly, multiply by another 1.33 – it turns out 100 mm.
We hope this is all clear, so further we will talk just about the effective focal length, and for SLRs we will indicate both numbers – it will be more convenient for everyone, and what you shoot with is completely unimportant. It’s just that in a DSLR you can always choose the appropriate lens, but for a compact it will be quite difficult to set the appropriate focal length – so do it at random. However, the article is addressed just to beginner “mirrors”.